The present invention relates to the susceptibility of electrical equipment to power line noise. More particularly, this invention relates to the testing for and correction of the susceptibility of a computer system to common mode noise on lines supplying power to the system.
Electrical power lines transmit electrical noise to equipment powered by the power lines. Once category of such noise is common mode noise. Common mode noise is a common mode voltage (CMV) or voltage that is present on both leads of an analog input with respect to an analog ground. Common mode voltage is different from normal mode voltage (NMV) which is the voltage that exists between the two leads of an analog input. As shown in FIG. 1, the most common source of common mode noise is the difference in potential between two physically remote grounds 102 and 104. In poorly grounded systems, this potential difference can raise both the hot and neutral power lines 106 and 108 to electrical equipment 110 up to about 30 volts above the house power supply ground 102. Even in properly grounded house power systems, this voltage differential may be in the neighborhood of 1 to 2 volts.
While ground potential differences are the major source of common mode noise, they are not the only source. Among the other causes of common mode noise are improperly grounded signals from other equipment tied to the power lines and radio frequency signals from sources such as microwave induction heaters and motor control relays. All the described types of noise signals result in ground current which could disrupt the operation of the equipment powered by the power supply. For instance, a computer ground current can cause damage to the computer circuitry or loss of data due to power supply voltage perturbations when power on reset of the power supply occurs.
Proper design and testing in the laboratory usually eliminates most power source problems. However, in-plant testing has not uncovered common mode noise conditions encountered in the field that have resulted in operational problems or damage to computer equipment. The common mode noise conditions encountered in the field can be continual or intermittent noise signals. In any case, the most desirable situation would be to be able to test in the laboratories for common mode failure conditions, particularly those failure conditions encountered in the field, to eliminate susceptibility to the noise.
Therefore, it is an object of the present invention to evaluate electrical equipment for common mode noise susceptibility.
Another object of the invention is to provide a new common mode noise testing to complement extensive normal mode noise testing now performed on computer power supply systems.
A further object of the invention is to provide method and apparatus to simulate common; mode voltage conditions encountered in the field under laboratory conditions to diagnose power failures and modify the systems to overcome those failures.
In accordance with the present invention, a test method and apparatus are provided to determine susceptibility of equipment to common mode noise problems. The equipment being tested is hooked up to a source of power with the power and neutral input terminals of the equipment isolated from the power source ground. Recorded common mode noise signals are then inserted between the equipment""s neutral input terminal and the power source ground so that the equipment is subject to the combination of the power source voltage and the recorded common mode noise signals to reproduce the common mode noise conditions occurring at the time of the power supply problem. When the problem condition is reproduced by the test signals, modification of the power supply for the equipment can be undertaken to eliminate the reoccurrence of the problem condition.
A library of recordings of common mode noise voltage patterns result in equipment problems can be accumulated. When new equipment is designed, it can be subjected to all or a selected subset of these recorded common mode noise patterns to enable the production of a more stable product. Common mode noise tests of a power supply can be performed in conjunction with normal mode noise tests to see what combinations of common mode and normal mode noise patterns induce problems. Also, the power supply tests can be performed simultaneously with the testing of the equipment deriving power from the power supply to determine how power supply noise conditions effect equipment operations.